(1) Field of the Invention
The present invention relates generally to paper machine operation and, more particularly, to systems and methods of analyzing and controlling paper machine operation in the formation section of a paper machine.
(2) Description of the Prior Art
Definitions
It is relevant and instructive to define those areas of a foil blade as done by the applicant for the purposes of describing the prior art and the present invention; these terms are as follows:
Activity zone or acceleration zone is the location on the blade where a change in direction is forced upon the fabric/sheet over an acceleration distance;
Approach angle is the angle at which the fabric/sheet enter the foil blade and/or acceleration zone moving in the machine direction;
Exit angle is the angle at which the fabric/sheet exit the foil blade and/or acceleration zone moving in the machine direction.
Drainage angle is the divergent angle that follows the flat surface that contacts the fabric/sheet; on a V blade it is the last divergent angle that follows the last flat surface. Drainage nip length is the sustended length of the drainage angle.
For example, in FIG. 1B of the prior art, activity zone 1 of foil blade B has an approach angle (θ1), an exit angle (α2), and an acceleration distance (F); also, foil blade B has a drainage angle (α2), and a drainage nip length (L).
Typically, foil blades are known to be used in the formation sections of paper machines and, in particular, are commonly employed on the wet end of Fourdrinier paper machines to extract water from the pulp fiber and water mixture or slurry and to induce activity of the sheet during its formation. The controlled extraction of water and the manipulation of activity levels of the sheet by using foil blades in the forming section of the paper machine are the preferred methods for controlling fiber distribution within the sheet. Additionally, manipulation of activity in the sheet can impact the quality of the finished paper sheet, most importantly in terms of its uniformity or formation.
Prior art foil blades commonly include a blade surface for contacting the sheet in the forming section of the paper machine; generally, the prior art blades are constructed so as to have a leading flat surface for contacting the sheet and the conveying fabric across the divergent surface so provided. This movement of the pulp sheet and the conveying fabric across at least one divergent surface introduced by the leading flat surface of the foil blade produces a vacuum effect on the sheet; it is this vacuum effect and the surface disruption created by the foil blade leading flat edge that are commonly recognized in the prior art to control the extraction of water and the sheet activity levels, thereby impacting the final paper sheet uniformity.
By way of further background of the prior art generally, the following brief description of the operational principles and features of conventional foil blades follows. FIG. 1A shows an arrangement of three conventional or prior art foil blade designs as they function on a paper machine interactively with the conveying fabric and sheet. The fabric travel direction is shown, also known as the machine direction. In each case, it is important to note that the foil blade has a flat leading edge that forms an angle greater than 90° with the paper sheet and conveying forming fabric as they move in the machine direction. Formerly prior art has not effected the use of the concept of “activity zones”; however, for the comparison with the present invention, the concept and related terminology set forth hereinabove is employed. The activity zones are identified with the dotted circles, Activity zone 1 and 2, respectively, on foil blade B; these activity zones are affected by the flat leading edge F and the entry and exit angles of the sheet and conveying fabric with respect to the horizontal, α2, θ1 and θ2, respectively, as shown in FIG. 1B, which is a close-up view of the foil blade B of FIG. 1A.
Importantly, the flat of the foil blade supports the conveying fabric and creates a water seal that enables vacuum to be generated and sustained by the motion of the conveying fabric and slurry over the divergent surface of the foil blade; thus, the leading flat edge of prior art foil blades is a critical feature to their function and operation. The water extracted from the sheet by the foil blade B is subsequently removed by a doctoring action of the foil blade C that immediately follows foil blade B in the machine direction.
Referring to FIG. 1B, discussing the general principles of operation of foil blades in the art using formulas and acceleration terminology developed and discovered by the applicant that are not taught in the prior art, foil blade B has a flat length (F), a divergent surface having an angle (α2) and such angle having a sustended length (L). The drainage from foil blade B is substantially proportional to the divergent angle (α2) and the sustended length of that angle (L). The activity imparted to the sheet by foil blade B is proportional to the acceleration of the fabric/sheet as it deflects and conforms to the surface of the foil blade. For example, referring again to FIG. 1B, the fabric/sheet approaches foil blade B at an approach angle (θ1), traverses the flat of foil blade B and diverges down the divergent surface at an angle (α2). The fabric leaves foil blade B and approaches foil blade C at angle (θ2). The conveying fabric/sheet experience an acceleration at two zones of activity as they traverse the foil blade B. The conveying fabric sheet enters the first activity zone at an approach angle (θ1), changes direction, and leaves the first activity zone at an exit angle (α2). This change in fabric direction takes place over a distance that is established by the length of the flat (F). Thus, the acceleration imparted to the sheet at activity zone 1 can be described by the following equation:Acceleration at zone 1=(fabric speed)2×(θ1+α2)/FSimilarly, the conveying fabric/sheet enters a second activity zone at an approach angle (α2) and leaves the second activity zone at an exit angle (θ2). Thus, the acceleration imparted to the sheet at activity zone 1 can be described by the following equation:Acceleration at zone 2=(fabric speed)2×(α1+θ2)/dwhere d is the distance over which the change in direction of the fabric/sheet takes place; this distance d is on the order of about ⅛ to about ½ inch.
It is the acceleration of the sheet at activity zones 1 and 2 of the foil blade B that determines the activity imparted to the sheet as it traverses foil blade B. FIG. 1B of the prior art shows activity zone 1 of foil blade B having an approach angle (θ1), an exit angle (α2), and an acceleration distance (F); also, foil blade B has a drainage angle (α2), and a drainage nip length (L). The drainage of the foil blade B is proportional to α2 and L. Conventional foil blade shapes such as those illustrated in FIGS. 1A and 1B have inherent drawbacks. For example, as shown in FIG. 1B, the exit angle of activity zone and the entry angle of activity zone 2 is the angle (α2), which is also the drainage angle of the foil blade. Thus, the primary foil blade angle that characterizes the activity of the foil blade is the same angle that characterizes the drainage of the foil blade; this linkage between activity imparted to a sheet and the drainage for a given foil blade is undesirable because it is not possible to affect changes to sheet activity and the drainage separately by modifying the foil blade. Often it is desirable to impart a substantial activity to the sheet without a corresponding increase in sheet drainage.
Additional relevant art includes methods for configuring forming sections on paper machines, more particularly, the activity of the paper sheet is assessed visually by a technician or service engineer, who may use a strobe light or the naked human eye. Various foil blades are installed and changes are made on a trial-and-error basis; if the sheet formation is improved, then additional changes consistent with the initial change may be made or if sheet formation is not improved, then other changes may be made. Notably, qualitative analysis focused on the overall foil box, not more detail. Because of the more linear nature of the forming section and related system, changes made at an upstream location on the paper machine in the forming section have a global effect at all locations downstream on the paper machine, which is why the trial-and-error modifications approach of the prior art is ineffective and often fails.
Thus, it is desirable to have the ability to analyze and control various parameters of a paper machine in its operation, in particular in the forming section of the paper machine, including but not limited to characteristics relating to foil blades wherein the activity and drainage characteristics associated with the blades, along with other parameters like sheet activity, sheet and fabric acceleration, sheet and fabric deflection, moisture profiles, and drainage, can be substantially separately and independently analyzed, established, and controlled.
Thus, there remains a need for a system and method to quantitatively analyze and control various parameters of a paper machine in its operation, in particular in the forming section of the paper machine, including but not limited to characteristics relating to foil blades wherein the activity and drainage characteristics associated with the blades, along with other parameters like sheet activity, sheet and fabric acceleration, sheet and fabric deflection, moisture profiles, and drainage, can be substantially separately and independently analyzed, established, and controlled on an individual foil blade basis.